If you are looking to block one or two specific users from your site, this feature is not available on your Yola site.
Yola does allow you to create a private website, or make part of your site private, by password protecting certain pages on your site.
We do have a Password Protection feature but this would apply to your site, no matter who visited. If this is, however, something you would want to do, you can find the steps on this link: Adding Password Protection.